Air cycle environmental control systems are well known. Such systems are commonly used on aircraft for conditioning air, which may include pressuring, cooling and dehumidifying the air, for delivery to the aircraft passenger cabin and crew cockpit for occupant comfort. The conditioned air may also be supplied to the equipment bay for cooling sensitive equipment.
Air cycle environmental control systems generally employ an air cycle machine comprised of a fan, a compressor, and one or more turbines mounted for rotation on a common shaft, the turbine or turbines driving both the compressor and the fan. The air cycle machine is typically powered by compressed air which is bled from the compressor section of an engine or an auxiliary power unit. The compressed bleed air is further compressed in the compressor of the air cycle machine and thereafter expanded through the turbine or turbines thereby cooling the air and generally condensing moisture therein. The bleed air is also typically cooled prior to compression by passing it in heat exchange relationship with cold air, most commonly ram air from the atmosphere outside the aircraft, drawn through the heat exchanger by the fan of the air cycle machine. The air may be further cooled between the compression and expansion steps by again passing it in heat exchange relationship with the cold ram air. This air that has been expanded through the turbine constitutes the conditioned air that is delivered to the aircraft cabin, cockpit and equipment bays.
In certain aircraft applications, such as for example military aircraft, it is known to integrate an air filtration system with the air cycle environmental control system in order to protect the occupants and equipment within the closed environment of the aircraft from contaminants in the atmosphere from which the air to be conditioned is drawn. Such filtration systems typically utilize filters, such as for example single stage regenerative bed filters designed to remove chemical, biological and/or nuclear contaminants from the conditioning air.
Since such filters have a limited filtration life unless periodically cleansed, it is customary to incorporate a pair of parallel filters into the filtration system, with one filter being cleansed by purge air, while the other filter is in operation. An air cycle environmental control system incorporating such an air filtration system is disclosed in commonly assigned U.S. Pat. No. 4,769,051. In the system disclosed therein, the purge air used to cleanse the filters is dumped directly overboard with the contaminants removed from the filters during the backflow cleaning cycle. Although the use of backflow purge air to cleanse the filters has proven very effective in extending the useful filtration life of the filters, there is an overall efficiency loss associated with the additional turbine back pressure resulting from the pressure drop the purge air experiences in traversing the bed being purged and due to the pressure drop resulting from a flow restriction added to the occupant cabin cooling circuit to balance the splitting of the conditioned air flow between the cabin cooling circuit and the parallel purge air flow circuit.